Memory cell and corresponding device

ABSTRACT

A radiation hardened memory cell includes an odd number of storage elements configured to redundantly store an input data logic signal. The storage elements include output lines for outputting respective logic signals having respective logic values. A logic combination network receives the respective logic signals and is configured to generate an output signal having a same logic value as a majority of the logic signals output from the storage elements. An exclusive logic sum circuit receives the respective logic signals output from the storage elements and is configured to produce a refresh of the logic data signal as stored in the storage elements when one of the logic signals output from the storage elements undergoes a logic value transition due to an error event.

PRIORITY CLAIM

This application claims priority from Italian Application for Patent No.102015000033005 filed Jul. 10, 2015, the disclosure of which isincorporated by reference.

TECHNICAL FIELD

The description relates to memory cells. One or more embodiments mayrelate to radiation hardened memory cells.

BACKGROUND

In certain environments, such as for example space environments, memorycells may be exposed to the radiation impact of heavy ions, which maylead to a so-called Single Event Effect or SEE.

Such events may adversely affect operation of memory cells that storelogical data for a long period, since phenomena such as SEE may lead tosuch data being modified.

There is a need in the art to counter such effects.

SUMMARY

One or more embodiments may be applied in memory cells adapted toreceive information from upstream circuits. A One Time Programmable ReadOnly Memory—OTP ROM, digital counters, shift registers,analog-to-digital converters are exemplary of such circuits.

One or more embodiments may offer one or more of the followingadvantages: very low “zero current” absorption, possibility ofdispensing with additional architectures such as a refresh oscillators,avoidance of noise possibly added by refresh oscillators, and smallersilicon area involved.

In an embodiment, a memory cell includes: a plurality of storageelements for redundantly storing an input data logic signal, saidplurality of storage elements having output lines for outputting arespective plurality of logic signals having respective logic values, alogic combination network sensitive to said respective logic values ofsaid logic signals on said output lines, said logic combination networkconfigured to generate an output signal of the cell having the samelogic value as a majority of said logic signals on said output lines,and an exclusive logic sum circuit sensitive to said respective logicvalues of said logic signals on said output lines, said exclusive logicsum circuit configured to produce a refresh of said data logic signal asstored in said plurality of storage elements, said exclusive logic sumcircuit activated when one of said logic signals on said output linesundergoes a logic value transition.

In an embodiment, said memory cell is part of an electronic device. Inan embodiment, a memory cell comprises: a plurality of storage elementsfor redundantly storing an input data logic signal, said plurality ofstorage elements having output lines for outputting a respectiveplurality of logic signals having respective logic values, and a sensingcircuit monitoring the plurality of logic signals to detect a singleevent transient (SET) error and in response thereto cause a refresh ofsaid data logic signal as stored in said plurality of storage elements.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments will now be described, by way of example only,with reference to the annexed drawings, in which:

FIG. 1 is a block diagram exemplary of a memory cell layout,

FIG. 2 is a detailed block diagram of one or more embodiments, and

FIGS. 3 and 4 are chronograms exemplary of signals which may occur in amemory cell of one or more embodiments.

DETAILED DESCRIPTION

In the ensuing description one or more specific details are illustrated,aimed at providing an in-depth understanding of examples of embodiments.The embodiments may be obtained without one or more of the specificdetails, or with other methods, components, materials, etc. In othercases, known structures, materials, or operations are not illustrated ordescribed in detail so that certain aspects of embodiments will not beobscured.

Reference to “an embodiment” or “one embodiment” in the framework of thepresent description is intended to indicate that a particularconfiguration, structure, or characteristic described in relation to theembodiment is comprised in at least one embodiment. Hence, phrases suchas “in an embodiment” or “in one embodiment” that may be present in oneor more points of the present description do not necessarily refer toone and the same embodiment. Moreover, particular conformations,structures, or characteristics may be combined in any adequate way inone or more embodiments.

The references used herein are provided merely for convenience and hencedo not define the scope of protection or the scope of the embodiments.

The block diagram of FIG. 1 is exemplary of a possible layout of amemory cell (for example flip-flop based) for electronic data with adata input D, a cell enable input EN and a clock input CK. Such anarrangement may be operable for writing data therein and making thosedata available as an output Q.

The deliberately simplified representation of FIG. 1 is generallyexemplary of such an arrangement including:

-   -   an input stage 1 which receives input data D,    -   an output stage 2 which receives data from the input stage 1 to        make them available as an output data Q, and    -   a further stage 3 configured to feed the output signal Q back to        the input of the output stage 2.

Operation of the stages 1, 2, and 3 (here simply represented as logicalinverters) is enabled by the enable signal EN and clocked by the clocksignal CK and its logical complement (CK(bar)) CK as generated by afurther inverter stage 4.

The general layout and principle of operation of the arrangement of FIG.1 is well known in the art which makes it unnecessary to provide a moredetailed description herein.

Briefly, when an arrangement as represented in FIG. 1 receives a clocksignal of for example “high” logical level, the data D is stored to bemade available as output Q. If the clock signal CK is set to a “low”logic level, whatever the input data D, the output Q maintains its value(that is information) at the state previously stored. Consequently, achange of the value of data may be admitted only on the front edge of ahigh logic level of the clock signal CK.

In certain environments, such as space environments, memory cells asschematically represented in FIG. 1 may be exposed to the impact ofheavy ions (for example a Single Event Effect or SEE) that may alter theinformation stored in the memory cell. This may possibly lead to a lossof information which in turn may cause incorrect operation of the memorycell and the devices in which such a memory cell is included, forexample with circuitry being enabled and/or disabled inappropriately.

The arrows in FIG. 1 are representative of various locations that may beaffected by a SEE, such as for example a Single Event Up Set or SEU.

For instance, a SEU may have an energy sufficient to induce a current ina PN junction and, if hitting for example the output of the stage 2 (forexample a bistable latch), the logic state stored at the output Q may bechanged (from 1 to 0 or from 0 to 1). Other circuit locations exposed toa SEU may include for example the input NET2 to the stage 2 or theoutput NET3 of the latch in the stage 3.

A solution to palliate the related negative events may involve resortingto redundancy, by storing identically the data in an (for example odd)plurality of latches (such as for example three latches) and then selectthe correct information as the information “voted” by the majority ofthe values latched.

Also, an event such as for example a Single Event Transient or SET canalter the logical state in the input stage 1 (for example a D flip-flop)so that incorrect information can be stored in a memory cell at theoccurrence of a clock pulse.

A possibility for palliating such a drawback may involve filtering theinput data D for example via a resistive-capacitive (RC) network with atime constant larger than the expected duration of the obnoxiousexternal event.

If occurring “upstream” of a clock signal, such an event may give riseto a glitch.

Again, a possibility of palliating such a drawback may lie in (low-passfor example RC) filtering the clock signal pulse by slowing it downappropriately for application to plural latches again to perform“voting” at the end of the data output.

If a SET occurs “upstream” of an enable signal EN, this may again giverise to a glitch with a momentary shutdown of the cell. Again, thepossibility exist of (low-pass for example RC) filtering adequately theenable pulse.

In cases where a memory cell as exemplified in FIG. 1 is used to storedata over a long time, even when resorting to a redundant structure, forexample to plural latches, different latches may be adversely affectedby subsequent SEU events, so that false information may be eventuallyproduced by the voting cell, due to the majority being possibly“reversed” since plural latches have been modified.

Continuous refresh of data may be resorted to in order to tackle thesesituations. Such a solution may result in an undesirable addition ofarchitectural components to the system for example a low-frequencyoscillator to generate a periodic clock pulse. This may increase powerconsumption and silicon area occupation; noise at the frequency of therefresh signal may also be generated which may adversely affect thesurrounding components.

Documents such as U.S. Pat. Nos. 7,212,056, 8,365,049 or 5,311,070 areexemplary of solutions proposed to counter such events.

FIG. 2 is a block diagram exemplary of one or more embodiments resortingto a redundancy scheme with a stage 10 including a plurality of storageelements 11, 12 and 13 (for example an odd number such as three) eachreceiving input data D1, D2, D3 as well clock signals CK1, CK2, CK3 andenable signals EN1, EN2, EN3.

The input data D1, D2, D3 to the various elements (for example latches)11, 12, 13 correspond to a “general” input data signal D which, in oneor mere embodiments, may be subjected to low-pass filtering (for examplevia an RC filter 20). In one or more embodiments the input signal D mayalso traverse a pass gate (for example a multiplexer) 80 to be describedin the following.

In one or more embodiments, low-pass filtering (for example in an RCfilter 30) may be applied also to the enable signal EN, which isdistributed to the enable inputs EN1, EN2 and EN3 of the latches 11, 12and 13.

Reference 40 denotes as a whole a “voting” cell including for examplethree AND gates 401, 402, 403 and an OR gate 404 which receives theoutputs from the AND gates 401, 402, 403 and produces a “general” outputsignal Q.

In or more embodiments, the cell 40 is intended to produce the output Qusing as an input the outputs Q1, Q2 and Q3 from the latches 11, 12 and13, so that Q has the same logic value as the majority of the logicsignals Q1, Q2 and Q3.

For instance:

-   -   if two of Q1, Q2 and Q3 are “1” and one is “0”, then Q is set to        “1”;    -   if two of Q1, Q2 and Q3 are “0” and one is “1”, then Q is set to        “0”.

In or more embodiments the input arrangement for the AND gates 401, 402,403 may be as follows:

-   -   input to the gate 401: the output signal Q1 from the latch 11        and the output signal Q2 from the latch 12;    -   input to the gate 402: output Q1 from the latch 11 and output Q3        from the latch 13;    -   input to the gate 403: output Q2 from the latch 12 and output Q3        from the latch 13.

Such an arrangement make it possible to produce a value for the output Qcorresponding to the “majority” of the values of Q1, Q2, and Q3. Forinstance if Q1=Q2=1 and Q3=0, the output for Q is set equal to 1.Conversely, if Q1=1 and Q2=Q3=0 the output value for Q is set to 0.

Those of skill in the art will appreciate that equivalent arrangementsfor the “voting” cell 40 may be devised.

In one or more embodiments, the outputs Q1, Q2 and Q3 from the threelatches 11, 12, 13 may also be input to an EX-OR (exclusive OR) gate 50whose output is fed (possibly after low-pass filtering at for example aRC filter 60) to one of the inputs of an OR gate 70, the OR gate 70receiving the clock signal CK as another input.

In one or more embodiments, the output signal Q may be fed back to thepass gate 80 so that either the low-pass filtered input signal D or theoutput signal Q may be input to the redundant arrangement including thelatches 11, 12, 13 under the control of signal SW.

In one or more embodiments, the data D input to the circuit (for examplefrom circuit 20) may be stored in the circuit upon receiving a clocksignal CK.

Once a steady state is reached the data is fixed at the output Q and,after a number of clock pulses, the pass gate 80 (originally intended tofeed the data D to the circuit) may switch to a condition where thesignal Q is fed back towards the latches 11, 12, 13 so that the upstreamcircuitry may (at least notionally) be turned off.

In one or more embodiments, generation and count of the signal SW maytake place externally, for example by means of a counter ormicroprocessor, with the signal SW input to the pass gate 80 as shown inFIG. 2.

In one or more embodiments, generation and count of the signal SW maytake place internally of the pass gate 80 for example by means ofcounters implemented in the pass gate 80: for instance a counter maycount, for example 16 clock pulses after which the pass gate 80 switchesto the condition where the output Q is fed back to the latches 11, 12,13.

One or more embodiments may thus create a clock pulse for driving thelatches 11, 12 and 13 only when an event (such as a SEE like a SingleEvent Transient or SET) occurs which modifies the data in one of theredundant flip-flops 11, 12 and 13.

In one or more embodiments, the EX-OR gate 50 is adapted to detect suchan occurrence.

The output data Q1, Q2 and Q3 of the flip-flops 11, 12, 13 are comparedin the gate 50. As long as Q1, Q2 and Q3 are at the same logical value,the output from the gate 50 is at a low value.

If the logical state of any of Q1, Q2, Q3 is changed, for example by aSET, the output from the gate 50 switches, for example to a high logiclevel thus generating a refresh pulse which (after possiblelow-filtering 60) is applied to the latches 11, 12, 13 via the gate 70.

The following is a truth table exemplary of possible values for Q1, Q2,Q3, the value Q corresponding to result of “voting” in the cell 40 andthe output from the EX-OR gate 50.

Those skilled in the art will promptly appreciate that other logicalnetworks other than those exemplified herein may be used to implementthe same truth table.

EX-OR OUT Q1 Q2 Q3 Q (gate 50) 0 0 0 0 0 0 0 1 0 1 0 1 0 0 1 0 1 1 1 1 10 0 0 1 1 0 1 1 1 1 1 0 1 1 1 1 1 1 0

The chronogram of FIG. 3 shows a possible time behavior (related to acommon time scale) of the clock signal CK, the switch signal SW, theinput data signal D and the output data signal Q exemplary of “regular”operation of the memory cell.

For instance, upon cell turn on, the input data D may be applied via themultiplexer 80 to the inputs of the latches 11, 12, 13 after low-passfiltering at 20 with a time constant larger than TIon (the expectedduration of disrupting event related e.g. to radiation impact of heavyions). A train of clock pulses CK may then “fix” the input D in thethree redundant memory cells (latches 11, 12 and 13) and the outcome of“voting” in the cell 40 may then be fixed as the general output Q.

As described previously, in one or more embodiments, an end count pulseSW may then cause the pass gate 80 to switch so that the output Q may bebrought to the inputs of the latches 11, 12, 13, and the circuitslocated upstream of the memory cell (such circuits not being visible inthe figures) may be disabled for example in order to avoid undesiredpropagation of noise and/or unnecessary power consumption.

In one or more embodiments, low-pass filtering (at 60) of the outputfrom the EX-OR gate 50 may avoid that, during initial operation asdescribed previously, any asymmetry in the responses of the latches 11,12, and 13 may undesirably lead to a lock status for the clock CK.

The diagrams of FIG. 4 are exemplary of the possible effect of forexample a heavy ion hitting one of the three latches 11, 12, 13 thusgiving a rise to a “collision” pulse I_COLL while the input data Dshould notionally remain at a constant value VD.

For instance, one may assume that the pulse I_COLL affects for examplethe first latch 11 by inducing a corresponding change in the voltage VQ1associated with the output Q1. Such a change in the logic condition oflatch will activate the EX-OR logic gate 50 by causing it to generate acorresponding clock pulse which, after low-pass filtering at 60 mayexhibit the time behavior exemplified as VCP in FIG. 4. Such a pulsewill propagate to the OR gate 70 and from here to the clock input CK1,CK2, CK3, so that a refresh pulse VCK from the gate 70 will cause thecorrect logical state VQ1 to be restored at the output Q1.

In that way the memory cell may be refreshed upon the occurrence of aSEE for example a SET, thus avoiding that plural events subsequentlyaffecting different ones of the storage elements 11, 12, and 13 may leadto a reversal of the majority in the voting cell 40.

One or more embodiments may thus include a plurality of storage elementsfor example an odd number such a three of storage elements such as forexample latches 11, 12, 13, which are intended to redundantly storeinput data logic signals D. The storage elements 11, 12, 13 may haveoutput lines for outputting a respective plurality of logic signals Q1,Q2, Q3 having respective logic values. A logic combination network (401to 404) sensitive to the respective logic values of these logic signalsQ1, Q2, Q3 may be configured for generating an output signal Q of thecell having the same logic value as a majority of these logic signalsQ1, Q2, Q3 on the output lines of the storage elements 11, 12, 13. Anexclusive logic sum circuit (for example an EX-OR gate such as 50) maybe sensitive to the respective logic values of the logic signals Q1, Q2,Q3 on the output lines of the storage elements 11, 12, 13 and beactivatable to produce refresh of the logic data signals D1, D2, D3 asstored in the storage elements 11, 12, 13 when one of these logicsignals Q1, Q2, Q3 undergoes a logic value transition due for example toa SEE such as a SET.

In one or more embodiments, the storage elements 11, 12, 13 may havecommon data inputs D1, D2, D3, common clock inputs CK1, CK2, CK3 and/orcommon enable inputs EN1, EN2, EN3.

In one or more embodiments, the common clock inputs CK1, CK2, CK3 may bedriven by the exclusive logic sum circuit 50, optionally through alow-pass filter 60.

In one or more embodiments, the common clock inputs CK1, CK2, CK3 may beconfigured for being driven alternatively, optionally via a logic sumgate 70 by:

-   -   an input clock signal CK, or    -   the exclusive logic sum circuit 50.

One or more embodiments may include low-pass filtering of:

-   -   a common data input D1, D2, D3 to the plurality of storage        elements 11, 12, 13 and/or    -   a common enable input EN1, EN2, EN3 to the plurality of storage        elements 11, 12, 13.

Without prejudice to the underlying principles, the details andembodiments may vary, even significantly, with respect to what has beendisclosed by way of example only without departing from the extent ofprotection.

The extent of protection is defined by the annexed claims.

The invention claimed is:
 1. A memory cell, comprising: a plurality ofstorage elements for redundantly storing an input data logic signal,said plurality of storage elements having output lines for outputting arespective plurality of logic signals having respective logic values, alogic combination network sensitive to said respective logic values ofsaid logic signals on said output lines, said logic combination networkconfigured to generate an output signal of the cell having the samelogic value as a majority of said logic signals on said output lines,and an exclusive logic sum circuit sensitive to said respective logicvalues of said logic signals on said output lines, said exclusive logicsum circuit configured to produce a refresh of said data logic signal asstored in said plurality of storage elements, said exclusive logic sumcircuit activated when one of said logic signals on said output linesundergoes a logic value transition.
 2. The memory cell of claim 1,wherein said plurality of storage elements includes an odd number ofstorage elements.
 3. The memory cell of claim 1, wherein said pluralityof storage elements comprise latch elements.
 4. The memory cell of claim1, wherein said storage elements include at least one of: common datainputs; common clock inputs; common enable inputs.
 5. The memory cell ofclaim 4, wherein said common clock inputs are driven by said exclusivelogic sum circuit.
 6. The memory cell of claim 5, further including alow-pass filter coupled between an output of the exclusive logic sumcircuit and the common clock inputs.
 7. The memory cell of claim 4,wherein said common clock inputs are configured for being drivenalternatively by a logic sum gate by: an input clock signal, or arefresh signal output from the exclusive logic sum circuit.
 8. Thememory cell of claim 1, wherein said exclusive logic sum circuitincludes an EX-OR gate sensitive to said logic signals on said outputlines of said storage elements.
 9. The memory cell of claim 1, furtherincluding a low-pass filter coupled at one location of: a common datainput to said plurality of storage elements, a common enable input tosaid plurality of storage elements.
 10. An electronic device including amemory cell, comprising: a plurality of storage elements for redundantlystoring an input data logic signal, said plurality of storage elementshaving output lines for outputting a respective plurality of logicsignals having respective logic values, a logic combination networksensitive to said respective logic values of said logic signals on saidoutput lines, said logic combination network configured to generate anoutput signal of the cell having the same logic value as a majority ofsaid logic signals on said output lines, and an exclusive logic sumcircuit sensitive to said respective logic values of said logic signalson said output lines, said exclusive logic sum circuit configured toproduce a refresh of said data logic signal as stored in said pluralityof storage elements, said exclusive logic sum circuit activated when oneof said logic signals on said output lines undergoes a logic valuetransition.
 11. The device of claim 10, wherein said device is one of: aread only memory, a digital counter, a shift register, ananalog-to-digital converter.
 12. A memory cell, comprising: a pluralityof storage elements for redundantly storing an input data logic signal,said plurality of storage elements having output lines for outputting arespective plurality of logic signals having respective logic values,and a sensing circuit monitoring the plurality of logic signals todetect a single event transient (SET) error and in response theretocause a refresh of said data logic signal as stored in said plurality ofstorage elements.
 13. The memory cell of claim 12, further comprising alogic combination network receiving said logic signals on said outputlines and configured to generate an output signal having a logic valuethat is the same as the logic value of a majority of said logic signalson said output lines.
 14. The memory cell of claim 12, wherein saidsensing circuit comprises an exclusive logic sum circuit receiving saidlogic signals on said output lines as inputs and having an outputconfigured to generate a refresh signal when one of said logic signalson said output lines undergoes a logic value transition.
 15. The memorycell of claim 14, further comprising a low pass filter circuit coupledbetween the output of the exclusive logic sum circuit and the pluralityof storage elements.
 16. The memory cell of claim 12, wherein saidplurality of storage elements share a common clock signal input, andwherein said sensing circuit generates a refresh signal applied to saidcommon clock signal input.
 17. The memory cell of claim 16, furthercomprising a logic OR gate having a first input receiving a clock signaland a second input receiving said refresh signal and having an outputcoupled to said common clock signal input.